The invention relates to apparatuses and methods for obtaining doses of flowable material, in particular doses of plastics, by cutting flowable material exiting from an extruding device. The doses thereby obtained can be processed by compression-moulding to obtain objects, particularly preforms for containers such as bottles.
The invention furthermore relates to apparatuses and methods for discarding possible defective doses so as to prevent such doses from being compression-moulded.
An apparatus is known for compression-moulding doses of plastics, comprising an extruding device from which the plastics exit along an exit direction. The apparatus furthermore comprises a cutting arrangement for separating the doses from the plastics exiting from the extruding device. The cutting arrangement comprises a plurality of knives rotatingly mobile around an axis parallel to the exit direction. Each knife is provided with a blade that extends on a plane perpendicular to the exit direction and keeps on this plane during rotation.
The plastics exit from the extruding device in a continuous manner. Thus whilst a knife is cutting a dose, the portion of plastics from which a part of the dose has already been separated continues to exit from the extruding device. This portion of plastics tends to stick to the blade, cooling prematurely, and exerts on the knife a thrust that may deform the blade, compromising cutting precision.
U.S. Pat. No. 4,640,673 discloses a particular type of apparatus for compression-moulding doses of plastics, which is provided with an extruding device and with a pair of knives rotatable around a rotation axis that cut the plastics exiting from the extruding device at preset intervals, so as to separate doses of plastics.
The knives rotate around the rotation axis at an angular speed that varies according to a preset law. In particular, the angular speed of the knives is relatively high when the knives separate the doses from the plastics exiting from the extruder. Immediately after cutting the doses, the angular speed of the knives is decreased. In order to move the knives at a variable angular speed, the apparatus disclosed in U.S. Pat. No. 4,640,673 comprises a pair of elliptical toothed wheels interposed between the knives and a motor that rotates the knives. The elliptical toothed wheels are designed in such a way as to ensure that the angular speed of the knives varies according to the desired law.
A drawback of the apparatus disclosed in U.S. Pat. No. 4,640,673 is that it is not very versatile. In fact, if it is desired to modify the law according to which the angular speed of the knives is varied, for example because the plastics to be extruded or the temperature thereof have been changed, or because it is desired to modify the length of the doses, it is necessary to dismantle the previously used pair of elliptical toothed wheels and replace the latter with a new pair of elliptical toothed wheels that drive the knives at the desired angular speed. This operation, in addition to requiring the arrest of the apparatus, is rather complicated and very time-consuming.
A further drawback of known apparatuses is that the cutting arrangement has surfaces to which the plastics, that are in a pasty state when they exit from the dispensing device, tend to adhere during cutting. These adhesion phenomena in fact impair the performance of a precise and clean cut of the plastics and lead to the creation of defective doses. Furthermore, the adhesion of the plastics, that are at a high temperature, causes significant wear to the knives, which thus have to be changed frequently, with consequent increase in the running costs of the apparatus.
U.S. Pat. No. 4,277,431 discloses a method for providing a measured quantity of extrudate material to a receiving arrangement, in which the extrudate material is severed by a flexible and resilient blade which contacts a cutting surface of an extrusion outlet during each revolution and bends from a rest condition to a backwardly bent flexed condition.
WO 2004/039553 discloses a device for removing doses of plastic material from an extruder by means of at least one removal element movable with respect to the extruder and provided with a receptacle for housing a removed dose. The removal element comprises a severing element adapted to cut the plastic material along a severing edge advancing through the material.
GB 2178359 discloses an apparatus comprising an extruder and a rotary cutting tool for cutting a synthetic resin in the heat-melted state extruded from the extrusion opening.
EP 1101587 discloses an apparatus for delivering plural plastic pellets into cavities for compression moulding of the pellets into bottle closures or bottle closure liners. The apparatus is provided for delivering molten plastic pellets from a supply of molten plastic to a plurality of moulding blocks, each moulding block having a row of cavities.
EP 1101586 discloses a rotary cutter separating a molten plastic pellet from a nozzle and depositing the molten plastic pellet into a mould cavity. A stream of air is radially directed along a cutter blade of the rotary cutter to displace a molten plastic pellet from the cutter blade into the mould cavity.
CH 443647 discloses a dispensing device for distributing doses of a mouldable plastic material, in which a jet of pressurized air is used in order to release a dose into a moulding cavity.